I'm trying to create an all-purpose portable storage media for booting images with grub4dos, keeping documents and applications etc. I will be working mostly with small files so an usb memory drive is out of discussion.

I would incline twords the second option because I like freedom, not being tied up to a specific hdd rack, like size, or capacity and stuff. Those that adata, wd and some other brands come with are virtually made to never replace the drives within. Which does not appeal to me.

I have one 2.5" adata 500 GB USB 2.0 it has a double USB connection to CPU so energy should not be a problem (when in use always pluged to MB ports), but always fails to connect to PC unless you boot the PC form internal HDD, with the external HDD already connected, I wasn't able to boot from this device since brand new: MBR Partitioned/formated 255H, 63S, tried from 1, 2, 3 or 4 primary partitions, (first active) in order to be under 130GB (limit for some Bios), never got it, so this is only used for storage.

Also I have 3 cabinets for 2.5" HDD's with double USB 2.0 connection to CPU, they never fail to connect to any PC, or boot from any of them, in MBR or UEFI Bios PC's.

As you can see all are USB 2.0, well there was not USB 3.0 yet at the time I bought them. (I'm thinking in buy some USB 3.0 cabinets now).

So I recommend you better assemble it yourself, before you buy anything, read carefully max. current (amps.) the cabinet has available for HDD and the selected HDD consumption in amps. some cabinets do not support HDD's bigger than 500 GB (because HDD high consumption in amps.).

Just remember Win7 and older can boot only from USB 2.0 (after some tricks), Win8 and newer are native USB 3.0 booting capable.